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Analysis of the Constituents of Coalbed Gas.

Since the early 1960's, the U.S. Bureau of Mines has maintained a comprehensive program of research into the occurrence, characteristics, and control of coalbed gas. Gas samples were collected at various underground and surface drill site locations in nearly every coal-producing area in the United States. Coalbed gas is generated as a byproduct of the coalification process. Throughout this process, chemical reactions occur at different temperatures and produce a range of gaseous byproducts. The evolved gases may escape or be partially captured in the coal or adjacent sratigraphic units. Geologic events following the formation of coal units can produce additional reactions which may alter the composition of the associated coalbed gas. A statistical analysis of 470 samples collected from 12 different coalbeds shows that methane is the primary constituent of coalbed gas, comprising 96.9 pct. The remaining hydrocarbons contribute about 0.2 pct. Nitrogen and carbon dioxide constitute the majority of the remaining gases at average concentrations of 1.4 and 1.2 pct. Respectively. In the Pittsburgh coalbed, a wide range of carbon dioxide concentrations was observed. This range of variation is about 12 pct.